Endpoint detection method and apparatus which utilize a chelating agent to detect a polishing endpoint

ABSTRACT

A method of planarizing a semiconductor wafer having a polishing endpoint layer that includes a ligand is disclosed. One step of the method includes polishing a first side of the wafer in order to remove the ligand from the wafer. Another step of the method includes determining that a chelating agent has bound the ligand due to the polishing step removing the ligand of the polishing endpoint layer. The method also includes the step of terminating the polishing step in response to determining that the chelating agent has bound the ligand. A polishing system is also disclosed which detects a polishing endpoint based upon a chelating agent binding a ligand of a polishing endpoint layer of a semiconductor device.

This application is a divisional of application Ser. No. 09/212,503,filed on Dec. 15, 1998 now U.S. Pat. No. 6,117,779.

BACKGROUND OF THE INVENTION

The present invention relates generally to an endpoint detection methodand apparatus, and more particularly to a method and apparatus thatutilize a chelating agent to detect a polishing endpoint of asemiconductor wafer.

Semiconductor integrated circuits are typically fabricated by a layeringprocess in which several layers of material are fabricated (i) on or ina surface of a wafer, or (ii) on a surface of a previous layer. Thisfabrication process very often requires layers to be fabricated upon asmooth, planar surface of a previous layer. However, the surfacetopography of layers may be highly uneven due to (i) areas which arehigher than the remainder of the surface or (ii) an uneven topography ofan underlying layer. As a result, a layer may need to be polished so asto present a smooth planar surface for the next processing step, such asformation of a conductor layer or pattern on the surface of anotherlayer.

In general, a semiconductor wafer may be polished to remove hightopography and surface defects such as crystal lattice damage,scratches, roughness, or embedded particles of dirt or dust. Thepolishing process typically is accomplished with a polishing system thatincludes top and bottom platens (e.g. a polishing table and a wafercarrier or holder), between which the semiconductor wafer is positioned.The platens are moved relative to each other thereby causing material tobe removed from the surface of the wafer. This polishing process isoften referred to as mechanical planarization (MP) and is utilized toimprove the quality and reliability of semiconductor devices. Thepolishing process may also involve the introduction of a chemical slurryto facilitate (i) higher removal rates, and (ii) selective removal ofmaterials fabricated upon the semiconductor wafer. This polishingprocess is often referred to as chemical mechanical planarization orchemical mechanical polishing (CMP).

In these polishing processes, it is often important to determine anendpoint of the polishing process. Over polishing (removing too much) ofa conductive layer results in increased circuit resistance and potentialscrapping of the semiconductor wafer. Since many processing steps haveoccurred prior to the polishing process, scrapping a semiconductor waferduring fabrication may result in a significant financial loss.Underpolishing (removing too little) of a conductive layer on the otherhand leads to failure in isolating circuits and results in electricalshorts, which leads to rework (redoing the CMP process) which raises thecost of production. Thus, a precise endpoint detection technique isneeded.

A typical method employed for determining endpoint in polishing systemsis to measure the amount of time needed to planarize a first wafer, andthen to run the remaining wafers for similar times. In practice thismethod is extremely time consuming, since operators must inspect eachwafer after polishing. The wafers must be inspected because the removalrate may vary during the polishing of an individual wafer, thus makingit extremely difficult to precisely control the polishing process.

Another method employed for determining the endpoint in polishingsystems is to (i) form a polishing endpoint layer in the semiconductordevice, and (ii) polish the semiconductor device down to the polishingendpoint layer. To this end, polishing systems detect when the polishingprocess reaches the polishing endpoint layer and terminate the polishingprocess in response to reaching the polishing endpoint layer. Varioustechniques have been used to detect when the polishing process reachesthe polishing endpoint layer. For example, U.S. Pat. No. 5,668,063 toFry et al. polishes a semiconductor device down to a tracer layer ofdetectable material. The polishing system of Fry determines that thetracer layer has been reached when a chemical element detector detectsmaterials such as boron or phosphorous of the tracer layer have beenremoved by the polishing process.

In order to base endpoint detection upon directly detecting material ofthe tracer layer, the chemical element detector needs to accuratelydetect rather small amounts of the tracer layer material, or thepolishing system needs to remove more of the tracer layer material inorder to provide the chemical element detector with enough material foraccurate detection. The above is also true if the material of the tracerlayer is consumed as a reagent of a chemical reaction to be detected bythe detector. In this case, the detector would need to be able to detectthe effect of a small reaction, or the polishing system would need toremove more of the tracer layer in order to provide enough tracermaterial for a substantial reaction to occur.

Detectors capable of detecting small amounts of the tracer layer ordetecting the effect of a small chemical reaction are more expensivethan detectors capable of detecting larger amounts of the tracer layeror detecting the effect of a larger chemical reaction. Furthermore, theadditional removal of the tracer layer in order to provide more tracerlayer material for direct detection increases the risk of overpolishingespecially when the topography of the tracer layer is highly uneven.

Thus, a continuing need exists for a method and an apparatus whichaccurately and efficiently detects when a polishing system polishes asemiconductor device down to a polishing endpoint layer.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, there isprovided a method of planarizing a semiconductor wafer having apolishing endpoint layer that includes a ligand. One step of the methodincludes polishing a first side of the wafer in order to remove theligand from the wafer. Another step of the method includes determiningthat a chelating agent has bound the ligand due to the polishing stepremoving the ligand of the polishing endpoint layer. The method alsoincludes the step of terminating the polishing step in response todetermining that the chelating agent has bound the ligand.

Pursuant to another embodiment of the present invention, there isprovided a method of planarizing a semiconductor wafer down to apredetermined distance from a semiconductor substrate of the wafer. Onestep of the method includes forming in the wafer a ligand that is at thepredetermined distance from the substrate of the wafer. Another step ofthe method includes polishing a first side of the wafer in order toremove the ligand from the wafer. The method also includes the step ofdetermining that a chelating agent has bound the ligand due to thepolishing step removing the ligand from the wafer. Moreover, the methodincludes the step of terminating the polishing step in response todetermining that the chelating agent has bound the ligand.

Pursuant to yet another embodiment of the present invention, there isprovided an apparatus for polishing a semiconductor wafer down to aligand of the wafer. The apparatus includes a polishing platen having apolishing surface, a wafer carrier, a slurry supply system, and apolishing endpoint detector. The wafer carrier is configured to (i)engage the wafer by the second side of the wafer and (ii) press thefirst side of the wafer against the polishing surface of the polishingplaten. The slurry supply system is configured to apply a chemicalslurry to the first side of the wafer which (i) facilities removal ofthe ligand from to the wafer, and (ii) receives the ligand removed fromthe wafer carrier. The polishing platen and the wafer carrier areconfigured to rub the first side of the wafer against the polishingsurface in the presence of the chemical slurry in order to remove theligand from the wafer. The polishing endpoint detector is operable todetect whether a chelating agent has bound the ligand due to thepolishing platen removing the ligand from the wafer. Moreover, thepolishing endpoint detector is operable to cause the polishing of thewafer to terminate in response to detecting that the chelating agent hasbound the ligand.

It is an object of the present invention to provide an improved methodand apparatus for determining an endpoint of a polishing process.

It is an object of the present invention to provide a new and usefulmethod and apparatus for determining an endpoint of a polishing process.

It is also an object of the present invention to provide a method andapparatus which accurately and efficiently detect when a polishingsystem polishes a semiconductor device down to a polishing endpointlayer.

It is a further object of the present invention to provide a method andapparatus for determining that a polishing system has polished a devicedown to a polishing endpoint without removing the device from thepolishing system.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description and theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F show sectional views of a semiconductor wafer during varioussteps of a fabrication process;

FIG. 2 shows an embodiment of a polishing system which incorporatesvarious features of the present invention therein; and

FIG. 3 shows a flowchart of a polishing procedure used by the polishingsystem of FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that there is no intent to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

As stated above, a semiconductor wafer after various steps of afabrication process needs to be planarized in order to remove roughtopography from the surface of the semiconductor wafer. FIGS. 1A-1Fillustrate sectional views of a semiconductor wafer 100 after varioussteps of a fabrication process of the present invention. In particular,FIGS. 1A-1F illustrate planarization of a semiconductor 100 down to apolishing endpoint layer 118.

FIG. 1A shows the semiconductor wafer 100 after a number of fabricationsteps. As shown, the semiconductor wafer 100 includes a semiconductorsubstrate 102 upon which a fabrication process formed a first insulatinglayer 104 and a first metal layer 106. In particular, the fabricationprocess formed the first insulating layer 104 upon the semiconductorsubstrate 102, and a contact hole 108 in the first insulating layer 104at a location above a transistor portion (not shown) of thesemiconductor substrate 102. Moreover, the fabrication process patternedthe first metal layer 106 (e.g. aluminum) over the first insulatinglayer 104 and the contact hole 108. As a result, the first metal layer106 fills the contact hole 108 forming an electrical contact with thetransistor portion of the semiconductor substrate 102, Moreover, thefilling of the contact hole 108 forms a pit 110 in the portion of thefirst metal layer 106 lying above the contact hole 108.

The semiconductor wafer 100 is shown in FIG. 1B after the fabricationprocess formed a second insulating layer 112 upon the surface of thesemiconductor wafer 100. As shown in FIG. 1B, the second insulatinglayer 112 has a highly uneven surface topography. The uneven surfacetopography of the second insulating layer 112 would cause accuracyproblems in fabricating additional layers of the semiconductor wafer100. For example, the uneven surface topography would cause controlproblems for a lithography process used to pattern a second metal layer120 (FIG. 1F) upon the second insulating layer 112. In order to avoidthe accuracy problems associated with the uneven topography of thesecond insulating layer 112, a polishing system such as the polishingsystem 210 of FIG. 2 polishes the second insulating layer 112 down to afirst level 116, thus planarizing the surface of the second insulatinglayer 112.

In order to enable the polishing system 210 of FIG. 2 to accuratelydetermine when the first level 116 is reached, the fabrication processimplants a polishing endpoint layer 118 into the second insulating layer112. As will be discussed in more detail below, the polishing endpointlayer 118 in the present invention includes a chelation ligand, such asplatinum ions, which the polishing system 210 uses in determining apolishing endpoint. As shown in FIG. 1C, the implanted polishingendpoint layer 118 starts at the first level 116 and ends at a secondlevel 120 that lies above the first metal layer 106. Methods of ionimplanting material into a semiconductor wafer are well known in theart. Accordingly, ion implantation techniques are not described herein.

However, it should be appreciated by those skilled in the art, thatimplantation techniques distribute the implanted material (e.g. platinumions) at various depths from the surface of implantation with a peakdistribution occurring at the desired depth. Accordingly, implantationtechniques implant some material at a depth greater than the desireddepth, and implant some material at a depth less than the desired depth.Since implantation of the material into underlying layers may result inreliability problems, the implantation technique should ensure that theunderlying layers such as the first metal layer 106 are not contaminatedwith the implanted material. Furthermore, since the polishing system 210utilizes the implanted material of the polishing endpoint layer 118 todetermine the polishing endpoint, the implantation technique shouldimplant the material of the polishing endpoint layer 118 such that thepolishing endpoint layer 118 starts below a second level 122 thatcorresponds to the lowest part of the surface to be planarized.

The implantation technique also tends to create a non-planar polishingendpoint layer 118 due to the uneven surface topography of the secondinsulating layer 112. Since the implantation technique implants thematerial of the polishing endpoint layer 118 through the surface of thesecond insulating layer 112, the implanted polishing endpoint layer 118generally follows the topography of the second insulating layer 112. Asa result, the implantation technique needs to account for the uneventopography of the second insulating layer 112 when determining thedesired depth of the implanted polishing endpoint layer 118. In somecases it may be desirable to polish the surface of the semiconductorwafer 100 prior to implanting the polishing endpoint layer 118 in orderto lessen variations in the surface topography of the second insulatinglayer 112. Polishing the second insulating layer 112 prior to implantingthe polishing endpoint layer 118 generally generates a more planarpolishing endpoint layer 118, thus making it easier to implant apolishing endpoint layer 118 which is (i) deep enough to cause thepolishing system 210 to planarize the semiconductor wafer 100, and (ii)not deep enough to cause the implantation technique to contaminateunderlying layers with implanted material.

FIG. 1D shows the semiconductor wafer 100 after the polishing system 210has polished the semiconductor wafer 100 down to the polishing endpointlayer 118. As depicted, the semiconductor wafer 100 has a planar surfaceupon which additional layers may be fabricated. For example, thefabrication process may etch a via 126 through the second insulatinglayer 112 and the polishing endpoint layer 118 (FIG. 1E). Then, thefabrication process may pattern a second metal layer 120 upon the secondinsulating layer 112 and the polishing endpoint layer 118 as shown inFIG. 1F. It should be appreciated that depending upon (i) the nature ofthe additional layer that the fabrication process is to form upon theplanarized surface of the semiconductor wafer 100 and (ii) the materialof the implanted polishing endpoint layer 118, the fabrication processmay need to form a third insulating layer upon the planarized surfaceprior to forming the additional layer in order to insulate theadditional layer from the polishing endpoint layer 118.

Referring now to FIG. 2, there is shown a preferred embodiment of apolishing system 210 which planarizes a front side or surface of asemiconductor wafer or device. To this end, the polishing system 210includes a platen motor or other drive mechanism 218 and platensubassembly 220. The platen motor 218 rotates the platen subassembly 220about a center axis 222 at a platen velocity V_(P). The platen motor 218may rotate the platen subassembly 220 in a clockwise direction as shownby arrow 224 or in the counterclockwise direction.

The platen subassembly 220 includes a polishing platen 226 and polishingpad 228 mounted upon the polishing platen 226. Both the polishing platen226 and the polishing pad 228 are preferably circular and define apolishing surface against which the polishing system 210 may polish thesemiconductor wafer 100. Moreover, the polishing pad 228 is typicallymade of blown polyurethane which protects the polishing platen 226 fromchemical slurry and other chemicals introduced during the polishingprocess.

The polishing system 210 also includes a polishing head subassembly 230.The polishing head subassembly 230 includes a wafer carrier 232, acooling mechanism 233, a carrier motor or other drive mechanism 234, anda wafer carrier displacement mechanism 236. The wafer carrier 232applies a controlled adjustable downward force F (as illustrated byarrow 238) to press semiconductor wafer 100 into polishing pad 228 tofacilitate polishing of the front side of the semiconductor wafer 100.The carrier motor 234 rotates wafer carrier 232 and semiconductor wafer100 about a center axis 237 at a wafer velocity V_(W). The carrier wafermay rotate the wafer carrier 232 in a clockwise direction as shown byarrow 235 or in the counterclockwise direction. However, the carriermotor 234 preferably rotates the wafer carrier 232 in the samerotational direction as platen motor 218 rotates the platen subassembly220 (although the carrier motor 234 may rotate the semiconductor wafer100 in the rotational direction opposite the rotational direction of theplaten subassembly 220 as desired).

The wafer carrier 232 also includes mechanisms (not shown) for holdingthe semiconductor wafer 100. For example, the wafer carrier 232 mayinclude a vacuum-type mechanism which generates a vacuum force thatdraws the semiconductor wafer 100 against the wafer carrier 232. Oncethe semiconductor wafer 100 is positioned on the wafer carrier 232 andheld in contact with the platen subassembly 220 for polishing, thevacuum force may be removed. In such an arrangement, the wafer carrier232 may be designed with a friction surface or a carrier pad whichengages the back side of the semiconductor wafer 100. Furthermore, thecarrier pad and downward force F create a frictional force between thewafer carrier 232 and the semiconductor wafer 100 that effectively holdsthe semiconductor wafer 100 against the wafer carrier 232 and causes thesemiconductor wafer 100 to rotate at the same velocity as the wafercarrier 232. Such wafer carriers and carrier pads are of conventionaldesign and are commercially available.

Moreover, the cooling mechanism 233 counteracts heat generated duringthe polishing process in order to maintain the wafer carrier 232 at asubstantially constant temperature. In particular, the cooling mechanismneutralizes the heat generated due to (i) friction and (ii) the chemicalslurry reacting with the front side of the semiconductor wafer 100.

The displacement mechanism 236 moves the wafer carrier 232 and thesemiconductor wafer 100 under a controlled force F across the platensubassembly 220 as indicated by arrows 231 and 239. The semiconductorwafer 100 is moved at an adjustable rate and along a variable polishingpath P. The polishing path P may be linear, sinusoidal, or a variety ofother patterns. The wafer carrier displacement mechanism 236 is alsocapable of moving the semiconductor wafer 100 along a polishing path toa location beyond the edge of the polishing pad 228 so that thesemiconductor wafer 100 “overhangs” the edge. This overhang arrangementpermits the semiconductor wafer 100 to be moved partially on andpartially off the polishing pad 228 to compensate for polishingirregularities caused by a relative velocity differential between thefaster moving outer portions and the slower moving inner portions of theplaten subassembly 220.

The polishing system 210 also includes a wash system 240 and a chemicalsupply system 260. The wash system 240 includes a wash reagent storage242, a wash reagent flow control mechanism 244, and a wash reagentconduit 246. The wash reagent storage 242 includes one or morecontainers for storing wash reagents. In particular, the wash reagentstorage 242 stores wash reagents which cause a chelating agent todisassociate with a ligand such as the metal ions implanted into thepolishing endpoint layer 118. Illustratively, the wash reagent storage242 may store ethanol (CH₃CH₂OH) which causes a chelating agent todisassociate with platinum ions of the polishing endpoint layer 118.

However, it should be appreciated that the above wash reagent is merelyexemplary and that many other wash reagents may be suitable for use withthe polishing system 210. For example, ethanol may be replaced withother wash reagents (e.g. other alcohols) as long as (i) the alternativewash reagent causes the chelating agent to disassociate with the ligandand (ii) the chelating agent and the ligand are soluble in thealternative wash reagent.

The wash reagent flow control mechanism 244 controls the flow of thewash reagent through the wash reagent conduit 246, and the wash reagentconduit 246 connects the wash system 240 to a chelating chamber 390 oftesting area 274 (discussed below) so that a wash reagent may betransferred from the wash system 240 to chelating chamber 390 of testingarea 274.

The slurry supply system 260 includes a slurry storage 262, a slurryflow control mechanism 264, and a slurry conduit 266. The reagentstorage 262 includes one or more containers for storing slurry. In apreferred embodiment, the slurry storage 262 stores (i) a slurry thatincludes alumina (i.e. AlO₃) which is an abrasive material thatfacilitates polishing of the front side of the semiconductor wafer 100and (ii) reactants which selectively react to certain materials of thefront side of the semiconductor wafer 100. Chemical slurries having suchproperties are well known and commercially available. Moreover, in thepreferred embodiment the slurry stored in the slurry storage 262 issufficiently acidic to dissolve the platinum ions of the polishingendpoint layer 118.

The slurry flow control mechanism 264 controls the flow of slurrythrough the slurry conduit 266, and the slurry conduit 266 transfers theslurry from the slurry storage 262 to the polishing area atop platensubassembly 220. To this end, the slurry flow control mechanism 264 andthe slurry conduit 266 introduce slurry as indicated by arrow 268 atopthe polishing pad 228 at a slurry flow rate Φ_(S).

The polishing system 210 further includes an endpoint detector assembly270. The endpoint detector assembly 270 includes an extraction conduit272, an extraction flow control mechanism 273, a testing area 274, asensor 276, and a waste conduit 277. Testing area 274 includes a numberof filters. Specifically testing area 274 includes an initial filter382, a secondary filter 384, and a chelating filter 386. Initial filter382, secondary filter 384, and chelating filter 386 divide testing area274 into an initial filtration chamber 380, a secondary filtrationchamber 388, a chelating chamber 390, and a terminal chamber 392. Theextraction conduit 272 receives effluent from the polishing area atopplaten subassembly 220. The effluent may include the chemical slurry ofthe slurry supply system 260, and materials removed from thesemiconductor wafer 100. The extraction flow control mechanism 273controls the flow of effluent from the extraction conduit 272 to theinitial filtration chamber 380 of testing area 274. To this end, theextraction flow control mechanism 273 and the extraction conduit 272introduce effluent to the initial filtration chamber 380 at anextraction rate Φ_(E). Moreover, the extraction flow control mechanism273 includes a pre-filter (not shown) which filters the aluminaparticles out of the effluent prior to the effluent entering initialfiltration chamber 380. In addition, the pre-filter removes anyrelatively large particles of various unknown solids from the effluent.

The waste conduit 277 of the endpoint detector 270 connects the terminalchamber 392 to a waste treatment facility (not shown). The waste conduit277 transfers the effluent at a waste flow rate Φ_(W) from the testingarea 274 to a waste treatment facility that treats and properly disposesof the effluent.

The sensor 276 is preferably an optical sensor, such as aspectrophotometer, operable to analyze the spectral characteristics ofthe effluent present in the chelating chamber 390 and generate anendpoint detection signal when the effluent has a particular spectralcharacteristic that is indicative of the polishing endpoint beingreached. To this end, the sensor 276 is operable to determine whether achelating agent present in chelating chamber 390 has formed a complex(i.e. chelated) with the ligand (i.e. platinum ions) of polishingendpoint layer 118. For example,5-[p-(dimethylamino)benzylidene]rhodanine, which has the followingformula:

can be utilized as the chelating agent in the present invention. Forinstance, a 0.2% 5-[p-(dimethylamino)benzylidene]rhodanine solution(weight/volume) can be prepared in 0.1% nitric acid (volume/volume), andthis chelating solution used to fill chelating chamber 390. It should beappreciated that 5-[p-(dimethylamino)benzylidene]rhodanine prepared inthe above described manner will chelate platinum ions, and the formationof the 5-[p-(dimethylamino)benzylidene]rhodanine:platinum ion complexcauses a color change in the solution contained within chelating chamber390. In particular, formation of the5-[p-(dimethylamino)benzylidene]rhodanine:platinum ion complex causesthe solution to change from a substantially clear solution to a solutionhaving a dark brown color.

However, it should be understood that chelating agents and ligands otherthan 5-[p-(dimethylamino)benzylidene]rhodanine and platinum ions,respectively, are contemplated as long as the alternative chelatingagent and ligand cause a spectral change, e.g. a color change, in thesolution contained within chelating chamber 390 when the chelatingagent:ligand complex is formed. For example, with respect to thechelating agent 5-[p-(dimethylamino)benzylidene]rhodanine, metal ionsother than platinum ions can serve as the ligand of polishing endpointlayer 118. In particular, silver, mercury, copper, gold, and palladiumions can be used as the ligand of polishing endpoint layer 118. Itshould also be understood that sensor 276 can also be aspectrophotometer operable to analyze spectral characteristics which arenot within the visible spectrum. For example, the sensor 276 can beoperable to analyze the ultraviolet or infrared spectral characteristicsof a solution containing a chelating agent and a ligand.

The sensor 276 is operable to detect the above described color change inthe solution contained within chelating chamber 390. Accordingly, thesensor 276 of endpoint detector 270 monitors the spectralcharacteristics of the solution contained within chelating chamber 390and generates an endpoint detection signal in response to detecting thatthe spectral characteristics (e.g. the color) of the solution haschanged in a predetermined manner.

The polishing system 210 further includes a controller 280 forcontrolling the polishing system 210 in order to effectuate the desiredpolishing results for the semiconductor wafer 100. The controller 280 isoperatively coupled to the components of the polishing system 210 viaconnectors 281-287 in order to monitor and control in real-time thecomponents of the polishing system 210. In particular, the controller280 is configured to control the advancement of the wash reagent intochelating chamber 390, the slurry flow rate Φ_(S), the extraction rateΦ_(E), the polishing path P, the wafer velocity V_(W), and the platenvelocity V_(P). The controller 280 may be implemented with amicrocontroller, an ASIC, discrete circuit components, or anycombination of the above. Furthermore, the controller 280 may includecomputational means for calculating specific parameters and memory forstoring software routines and parameters.

More specifically, the controller 280 is coupled to the wash reagentflow control mechanism 244 via a connector 281 to control theadvancement of the wash reagent into chelating chamber 390. Thecontroller 280 is coupled to the displacement mechanism 236 viaconnector 282 to monitor and controllably adjust the polishing path P ofthe semiconductor wafer 100 and the speed at which the semiconductorwafer 100 is moved across the platen subassembly 220. The controller 280is coupled to the carrier motor 234 via connector 283 to monitor themotor rpm and wafer velocity V_(W) imparted by the wafer carrier 232 andto adjust the speed of the wafer carrier 232 as desired. The controller280 is coupled to platen motor 218 via connector 284 to monitor themotor rpm and platen velocity V_(P) of platen subassembly 220, and toadjust the speed of the platen subassembly 220 as desired. Thecontroller 280 is coupled to the extraction flow control mechanism 273via a connector 285 to monitor and adjust the flow rate Φ_(E) of theeffluent. The controller 280 is coupled to the sensor 276 of theendpoint detection system 270 via a connector 286 to monitor theendpoint detection signal generated by the endpoint detection device270. The controller 280 is coupled to the slurry flow control mechanism264 via a connector 287 to monitor and adjust the slurry flow rate _(S).

Prior to operating the polishing system 210, a fabrication process formsin the semiconductor wafer 100 a polishing endpoint layer 118 thatincludes a chelation ligand (e.g. platinum ions). In particular, thefabrication process forms the polishing endpoint layer 118 at apredetermined distance from the surface of the semiconductor wafer 100,the substrate 102, or another layer of the semiconductor wafer 100.(Note that the aforementioned predetermined distance can take intoaccount any “braking distance tolerance” required by polishing system210, e.g. any finite amount of polishing which takes place after theendpoint signal (discussed below) has been generated.) Then, the wafercarrier 232 engages the back side of the semiconductor wafer 100 andpresses the front side of the semiconductor wafer 100 against thepolishing pad 228 with a force F. The controller 280 causes (i) theplaten motor 218 to rotate the platen subassembly 220 at a platenvelocity V_(P), (ii) the carrier motor 234 to rotate the wafer carrier232 at a wafer velocity of V_(W), (iii) the displacement mechanism 236to execute a polishing path P, and (iv) the slurry flow controlmechanism 264 to apply chemical slurry to the polishing pad 228 at aslurry flow rate  _(S). The resulting complex movement of the wafercarrier 232 relative to the polishing pad 228, the force F, and thechemical slurry all cooperate to remove material, including the ligand,from the front side of the semiconductor wafer 100.

Once the polishing system 210 is close to the polishing endpoint layer118, the controller 280 causes the extraction flow control mechanism 273to extract effluent from the polishing pad 228 at an extraction rateΦ_(E). Extraction flow control mechanism 273 then advances the effluentinto initial chamber 380 via conduit 304. It should be understood thatthe effluent is advanced into initial chamber 380 under enough pressureso that the effluent is force through initial filter 382. The size ofthe pores defined in initial filter 382 are such that any smallparticles which escaped filtration by the pre-filter are trapped oninitial filter 382. Preferably, initial filter 382 is a 5 micron teflonfilter available from the Millipore Corporation located in Bedford,Mass. After passing through initial filter 382, the effluent enterssecondary filtration chamber 388 and is forced through secondary filter384. The size of the pores defined in secondary filter 384 are such thatrelatively smaller particles which escaped filtration by the pre-filterfilter and initial filter 382 are trapped on secondary filter 384.Preferably, secondary filter 384 is a 1 micron teflon filter which canbe obtained from the Millipore Corporation located in Bedford, Mass.After passing through secondary filter 384, the effluent enterschelating chamber 390. Once in chelating chamber 390, the effluent ismixed with the chelating solution that contains5-[p-(dimethylamino)benzylidene]rhodanine. It should be appreciated thatif platinum ions of the polishing endpoint layer 118 are present in theeffluent that enters chelating chamber 390, the above discussed5-[p-(dimethylamino)benzylidene]rhodanine:platinum ion complex isformed. Forming the 5-[p-(dimethylamino)benzylidene]rhodanine:platinumion complex causes the effluent contained within chelating chamber 390to change from a substantially clear solution to a solution having adark brown color. The endpoint detector 270 then generates an endpointsignal once the endpoint detector 270 detects the aforementioned changein the spectral characteristic of the effluent has occurred. In responseto the endpoint detector 270 generating the endpoint signal, thecontroller 280 terminates the polishing of the semiconductor wafer 100since the semiconductor wafer 100 has a planar surface with an eventopography when the polishing system 210 polishes the semiconductorwafer 100 down to the polishing endpoint layer 118.

FIG. 3 illustrates a polishing procedure 300 utilized by polishingsystem 210. The controller 280 in step 310 of the polishing procedure300 causes the polishing system 210 to begin polishing the front side ofthe semiconductor wafer 100 in order to remove material from the frontside of the semiconductor wafer 100. To this end, the controller 280causes the wafer carrier 222 to rub the front side of the semiconductorwafer 100 against the platen subassembly 220. Moreover, the controller280 causes the chemical slurry supply system 260 to apply chemicalslurry atop platen subassembly in order to facilitate the removal ofmaterial from the front side of the semiconductor wafer 100.

After a predetermined amount of polishing time has elapsed, thecontroller in step 330 causes the extraction flow control 273 of theendpoint detector 270 to introduce effluent extracted from the platensubassembly 220 into the initial filtration chamber 380 via conduit 304.The predetermined amount of polishing time is close to but less than theamount of polishing time required for the polishing system 210 to reachthe polishing endpoint layer 118. The endpoint detector 270 thenanalyzes in step 340 the effluent in order to determine whether the5-[p-(dimethylamino)benzylidene]rhodanine:platinum ion complex has beenformed.

Since in the preferred embodiment the chelating agent5-[p-(dimethylamino)benzylidene]rhodanine reacts with metal ions, inparticular platinum ions, to produce a spectral characteristic change inthe effluent present in the chelating chamber 390, the sensor 276 of thepreferred embodiment in step 350 determines whether the effluent inchelating chamber 390 changes from a substantially clear solution to asolution have a dark brown color. If sensor 276 determines that thespectral characteristic has occurred, then the sensor 276 transmits anendpoint signal to the controller 280; otherwise, the polishing system210 returns to step 330 to extract more effluent.

In response to receiving the endpoint signal, the controller 280 in step360 causes the polishing system 210 to stop polishing the semiconductorwafer 100. It should be appreciated that the controller 280 may allowthe polishing system 210 to continue polishing the semiconductor wafer100 for a predetermined amount of time in order to further removematerial from the semiconductor wafer 100. This further removal ofmaterial or overpolishing may be desirable after certain steps of afabrication process.

The controller 280 then in step 370 flushes the platinum ions from thepolishing platen 220. In particular, the controller 280 causes thechemical slurry to be applied to the polishing platen 220 for apredetermined period of time. The continual application of the chemicalslurry after the polishing system 210 has stopped polishing thesemiconductor wafer 100 helps flush the platinum ions of the polishingendpoint layer 118 from the platen subassembly 220 so that the endpointdetector 270 may detect the endpoint of the next semiconductor wafer tobe polished.

In addition, in step 370, the controller 280 causes the wash reagentflow control mechanism 244. to advance a volume of the wash reagent tochelating chamber 390 via wash reagent conduit 246. Advancing a volumeof the wash reagent into chelating chamber 390 causes (i)5[-p-(dimethylamino)benzylidene]rhodanine to disassociate with theplatinum ions of the polishing endpoint layer 118 and (ii) the platinumions to be washed out of chelating chamber 390. Specifically,substantially all of the platinum ions are advanced through the poresdefined in chelating filter 386 and into terminal chamber 392. Oncelocated in terminal chamber 392, the platinum ions are advanced to thewaste treatment facility via waste conduit 277. However, it should beunderstood that the pores defined in chelating filter 386 are of thesize such that the chelating agent, i.e.5-[p-(dimethylamino)benzylidene]rhodanine, is substantially preventedfrom passing through chelating filter 386. On the other hand, asdiscussed above, the pores defined in chelating filter 386 are largeenough such that particles which are smaller than the chelating agent(e.g. a chelating ligand such as platinum ions) can pass throughchelating filter 386 and thus be removed from chelating chamber 390.Preferably, chelating filter 386 has a pore size up to 10 nanometers. Itshould be noted that The Center for Micro-Engineered Materials at theUniversity of New Mexico located in Albuquerque, N.M. fabricates variousfilters similar to chelating filter 386. Thus, a chelating filtersuitable for use as chelating filter 386 may be obtained upon requestfrom The Center for Micro-Engineered Materials. Therefore, it should beappreciated that the chelating agent substantially remains withinchelating chamber 390, secondary filtration chamber 388, and initialfiltration chamber 380. Note however that the chelating agent primarilyremains in chelating chamber 390. It should also appreciated thatcausing the platinum ions to disassociate from the chelating agentresults in the solution in chelating chamber 390 turning from a darkbrown color back to a substantially clear solution. Once the platinumions have disassociated from the chelating agent, and the solution inchelating chamber 390 has turned back to a substantially clear solution,the above described polishing process can be repeated with anothersemiconductor wafer 100.

It should be appreciated that, as a result of using a chelating agent,only a small amount of ligand (e.g. platinum ions) from the polishingendpoint layer 118 is required to cause a significant spectral change inthe solution contained in chelating chamber 390. Therefore, utilizing achelating agent which causes a significant spectral change of a solutionwhen in the presence of a small amount of ligand effectively amplifiesthe presence of the ligand. This amplification results in a detectionsystem that can utilize a relatively inexpensive spectrophotometer typedetector to indirectly detect the presence of very small amounts ofligand (e.g. platinum). This is in contrast to other polishingarrangements which utilize detection methods which attempt to directlydetect material removed from the polishing endpoint layer 118. Thesepolishing arrangements must utilize relatively expensive detectors whichincreases the cost of manufacturing semiconductor wafers.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description isto be considered as exemplary and not restrictive in character, it beingunderstood that only a preferred embodiment has been shown and describedand that all changes and modifications that come within the spirit ofthe invention are desired to be protected. For example, while thepolishing endpoint layer 118 has been described as being implanted intothe semiconductor wafer 100, other methods of forming the polishingendpoint layer 118 may also be used. For example, U.S. Pat. No.5,668,063 to Fry et al., the disclosure which is hereby incorporated byreference, discloses forming a tracer layer upon other layers. Thismethod of forming a tracer layer may be suitable for forming thepolishing endpoint layer 118 used by the polishing system 210 of thepresent invention.

What is claimed is:
 1. An apparatus for polishing a semiconductor wafer down to a ligand of said wafer, said wafer having a first side and a second side, comprising: a polishing platen having a polishing surface; a wafer carrier configured to (i) engage said wafer by said second side of said wafer and (ii) press said first side of said wafer against said polishing surface of said polishing platen; a slurry supply system configured to apply a chemical slurry to said first side of said wafer which (i) facilities removal of said ligand from said wafer, and (ii) receives said ligand removed from said wafer, wherein said polishing platen and said wafer carrier are configured to rub said first side of said wafer against said polishing surface in the presence of said chemical slurry in order to remove said ligand from said wafer; and a polishing endpoint detector that is operable to (i) detect whether a chelating agent has bound said ligand due to said polishing platen removing said ligand from said wafer, and (ii) cause said polishing of said wafer to terminate in response to detecting said chelating agent has bound said ligand.
 2. The apparatus of claim 1, wherein said polishing endpoint detector comprises: a slurry extractor that is operable to extract said chemical slurry from said polishing platen.
 3. The apparatus of claim 2, further comprising: a number of filters which are configured to serially filter said chemical slurry extracted from said polishing platen.
 4. The apparatus of claim 2, further comprising: a spectrophotometer which is operable to determine whether said chelating agent has bound said ligand.
 5. The apparatus of claim 4, wherein: said spectrophotometer detects a color change in a solution which contains said ligand and said chelating agent.
 6. The apparatus of claim 5, wherein: said chelating agent is 5-[p-(dimethylamino)benzylidene]rhodanine, and said ligand is selected from a group of ions comprising platinum ions, silver ions, mercury ions, and palladium ions.
 7. The apparatus of claim 1, further comprising: a wash reagent supply system which is operable to add a wash reagent to a solution containing said ligand and said chelating agent such that a complex of said ligand and said chelating agent disassociates. 